Many intrusion detection barriers based on the use of optical fibers have been described in the prior art. In one type of such barriers, a sensor wire comprising optical fibers is stretched horizontally under tension between posts and is connected at one to an optical transmitter and at the other end to an optical receiver. Any attempt to climb over the fence results in changes of tension and possibly in damage to the sensor wire, and therefore in a change in the intensity of the light transmitted through it, which is sensed by the optical receiver, and activates an alarm.
A security fence comprising optical fibers is described in U.S. Pat. No. 4,777,476. It comprises a multiplicity of hollow rigid bar elements and an optical fiber extending through some of the bar elements. Predetermined bending of the optical fiber is provided in response to bending of some of the rigid bar elements by a given amount. An optical fiber support is disposed within each of the bar elements containing the optical fiber and arranged so as not to be displaced in response to bending of the corresponding rigid bar element up to a given amount. Such a structure however requires the use of hollow rigid bars, is not adapted to standard fences and is suited only to special applications, and further, is highly expensive.
Another type of a known security fence, described in European Patent 49,979, a mesh structure is provided, comprising an upper and a lower horizontal wire and transverse wires attached to them and disposed at a slant, to cross one another. All the wires are made of or comprise optical fibers. The various optical fibers are connected at their jointing points by means of connecting members which prevent relative displacement of said fibers and are sufficiently positive to ensure damage to the optical fibers when a certain load is applied to the mesh. Each fiber is connected to an optical transmitter and an optical receiver, so that its rupture will cause interruption of light transmission between the two and activate an alarm. Such a wire fence requires a multiplicity of optical circuits, each consisting of an optical transmitter, an optical receiver, and the fiber connecting them, which constitutes a disadvantage. Furthermore, it does not provide full protection against an intrusion, because an intruder may carefully cut through the connecting members placed at the joints between optical fibers, along a plane substantially parallel to the plane of the fence, and thus free the optical wires from the mutual, rigid connection on which the operation of the fence depends; and it is sensitive to false alarms caused e.g. by animals pushing against the fence.
Prior U.S. Pat. No. 4,399,430 - Kitchen - relates to a security fence intended for the same purpose and which also includes optical fibers, means for transmitting light through these and means for detecting a change of light intensity due to mechanical forces applied to the fence structure. The Kitchen structure is a woven structure, defined by a Textile Dictionary as "a structure composed of two interlaced materials in the warp and weft directions" (In Kitchen: "elongated members"). Woven structures require the interlacings of two components: warp and weft, with a plurality of strands in each direction.
Contrary to common woven textile products, the spacings between the strands in Kitchen's Patent are large, not jammed next to each other and therefore are not fixed in space, which makes it possible to move the strands of the warp and the weft in the direction of the application of force.
Kitchen inserts ferrules, which can be encapsulated, at the intersections so that they prevent a movement of the strands. This is imperative for the Kitchen fence as otherwise such a fence is easily penetrated.
Scrutiny of Kitchen's Patent reveals a serious flaw: his FIGS. 1, 2, 3, 3a and 4 describe such a woven structure, whereas Kitchen's FIGS. 5, 6 and 6a are not related to the woven structure, and no teaching is provided how such joining points can be produced in a woven structure. According to expert opinion, the joining points of his FIGS. 5, 6 and 6a are feasible in a woven structure. Kitchen comments on the superiority of such joinings but does not demonstrate how these can be produced. It seems that these are inoperable and were included in an attempt to cover all possibilities. It is not possible to produce Kitchen's woven structure from a single strand. This is an inherent feature of his structure.
A structure with such "interweaving" joints as shown in said patent, can be produced by either weaving or by unique braiding.
When such a structure is produced by weaving it will require two systems of strands, called warp and weft, with multiple intersecting strands in each. A woven structure used for the purpose of constructing a security fence with optical wires, will require a multitude of Transmitter/receiver units.
When the said structure is produced by braiding it will require only one warpwise system of strands which will be used for the warp, but also for the weft. As the structure is mounted on a frame in a diagonal configuration, each strand reaches the edge of the structure, interlaces around the frame and changes direction, to create the weft. Each strand is used in a "serpentine" manner, and appears again and again in different sections of the structure. The number of strands needed to produce the braided structure depend on the width of the structure (used as the height of the fence), and the wider the fence needed the more numerous the number of strands, and also the number of transmitter/receiver units needed. In order to prevent intruder penetration, the intersections must be strengthened by rigid elements, such as ferrules.
Another drawback of the known security fences based on optical fibers, is that they will not respond to sound an alarm unless the fibers have been cut or deformed to a degree which requires that a very high load be placed on them. If the fiber is not cut but deformed, the transmission of light therethrough will not completely cease, but will be reduced; however, a reduction sufficient to cause the system to respond will only be produced beyond a high deformation threshold. For this reason it has been suggested in the prior art, to provide the fence with auxiliary devices, such as the rigid joints of the cited European Patent, which will cause damage to the fiber and sharply reduce the light transmission through them. However, the need for such auxiliary devices is a drawback, and further, they can be cut and neutralized.